U.S. patent number 6,524,664 [Application Number 09/374,347] was granted by the patent office on 2003-02-25 for photocatalytically hydrophilifying and hydrophobifying material.
This patent grant is currently assigned to Toto Ltd.. Invention is credited to Akira Fujishima, Kazuhito Hashimoto, Makoto Hayakawa, Mitsuhide Shimohigoshi, Toshiya Watanabe.
United States Patent |
6,524,664 |
Hashimoto , et al. |
February 25, 2003 |
Photocatalytically hydrophilifying and hydrophobifying material
Abstract
A method for highly hydrophilifying the surface of an article by
photoexcitation of a semiconductor photocatalyst and maintaining
the hydrophilicity is disclosed. A layer containing a photocatalyst
is formed on a substrate. Onto the surface of the layer are fixed a
hydroxyl group upon photoexcitation of the photocatalyst and a
physically adsorbed water molecule in the vicinity of the hydroxyl
group upon photoexcitation of the photocatalyst. Thus, the surface
is highly hydrophilified. Further, this surface, simultaneously
with the hydrophilification, exhibits higher hydrophobicity.
Inventors: |
Hashimoto; Kazuhito (Yokohama,
JP), Fujishima; Akira (Kawasaki, JP),
Watanabe; Toshiya (Kitakyushu, JP), Shimohigoshi;
Mitsuhide (Kitakyushu, JP), Hayakawa; Makoto
(Kitakyushu, JP) |
Assignee: |
Toto Ltd. (Fukuoka-ken,
JP)
|
Family
ID: |
27334495 |
Appl.
No.: |
09/374,347 |
Filed: |
August 13, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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987670 |
Dec 9, 1997 |
5939194 |
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933886 |
Sep 19, 1997 |
6013372 |
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PCTJP9600733 |
Mar 21, 1996 |
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Foreign Application Priority Data
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Dec 9, 1996 [JP] |
|
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8-344584 |
Sep 4, 1997 [JP] |
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9-256090 |
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Current U.S.
Class: |
427/558;
427/126.3; 427/553; 427/595; 427/377 |
Current CPC
Class: |
C03C
17/2456 (20130101); G02B 1/105 (20130101); G02B
1/18 (20150115); C03C 17/256 (20130101); C09K
3/18 (20130101); Y10T 428/31504 (20150401); C03C
2217/71 (20130101); C03C 2218/113 (20130101); C03C
2217/23 (20130101); C03C 2217/212 (20130101); C03C
2218/151 (20130101) |
Current International
Class: |
C03C
17/25 (20060101); C03C 17/245 (20060101); C03C
17/23 (20060101); C09K 3/18 (20060101); G02B
1/10 (20060101); B05D 003/06 () |
Field of
Search: |
;428/411.1,702,688,689,704 ;427/126.3,558,377,553,595 |
References Cited
[Referenced By]
U.S. Patent Documents
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0433915 |
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0590477 |
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1022588 |
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149281/78 |
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61-83106 |
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61-91042 |
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63-100042 |
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1288321 |
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3-101926 |
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4-174679 |
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6-278241 |
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6298520 |
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6315614 |
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7051646 |
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1218635 |
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7113272 |
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8119673 |
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7171408 |
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JP |
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60-221702 |
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Nov 1995 |
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JP |
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8313705 |
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Nov 1996 |
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JP |
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9-227158 |
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Sep 1997 |
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JP |
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WO9511751 |
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May 1995 |
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WO |
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Primary Examiner: Barr; Michael
Assistant Examiner: Jolley; Kirsten Crockford
Attorney, Agent or Firm: Jones, Day, Reavis & Pogue
Parent Case Text
This application is a continuation of application Ser. No.
08/987,670, filed Dec. 9, 1997, now U.S. Pat. No. 5,939,194, which
is a continuation-in-part of application Ser. No. 08/933,886, filed
Sep. 19, 1997, now U.S. Pat. No. 6,013,372, which is a
continuation-in-part of PCT Application No. JP96/00733, filed Mar.
21, 1996, and which designated the U.S.
Claims
What is claimed is:
1. A method comprising: starting exposure of a photocatalytic
coating layer on a substrate to photoexcitation, said layer
including a photocatalyst; and stopping said exposure when said
photoexcitation yields a mosaic of hydrophilic and hydrophobic
regions in the exposed area.
2. The method of claim 1 wherein each of said hydrophilic and
hydrophobic regions have an area of about 100 to 10,000
nm.sup.2.
3. The method of claim 1 wherein, after said stopping step, the
surface of said layer is amphiphilic such that oil deposited on
said surface can be easily removed by rinsing said surface with
water and water deposited on said surface can be easily removed by
rinsing with oil.
4. The method of claim 1 wherein said hydrophobic regions are more
hydrophobic after said stopping of said exposure than before said
starting of said exposure.
5. The method of claim 1 wherein the total area of said hydrophilic
regions is at least about 20% but less than 100% of said exposed
area.
6. The method of claim 1 wherein, after said stopping step, the
surface of said layer has a contact angle with water of not more
than 5.degree. and a contact angle with glyceride trioleate of not
more than 5.degree..
7. The method of claim 1 wherein the photoexcitation is carried out
by exposing the layer to sunlight or UV light having an intensity
within the range from 0.001 to 1 mW/cm.sup.2.
8. The method of claim 1 wherein said photocatalyst comprises a
metal oxide, and oxygen atoms of said metal oxide are exposed on
the surface of said layer, and substantially all of said exposed
oxygen atoms are oxygen atoms at bridging sites in the metal
oxide.
9. The method of claim 8 wherein the metal oxide is rutile titanium
oxide with the (110) crystal face or (100) crystal face thereof
being exposed on the surface of the layer containing a
photocatalyst.
10. The method of claim 8 wherein the metal oxide is anatase
titanium oxide with the (001) crystal face thereof being exposed on
the surface of the layer containing a photocatalyst.
11. The method of claim 1 further comprising the step of subjecting
the layer to water.
12. The method of claim 11 wherein the water is in the form of
humidity.
13. The method of claim 1 wherein said hydrophilic regions have
hydroxyl groups fixed thereto, with water molecules physically
adsorbed to said hydroxyl groups.
14. A method of increasing hydrophobicity of a hydrophobic region
on a photocatalytic coating layer, the method comprising: starting
exposure of the region to photoexcitation; and stopping said
exposure when said photoexcitation renders the exposed region more
hydrophobic than before the starting of said photoexcitation.
15. The method of claim 14 wherein the increase in hydrophobicity
is characterized by a contact angle of the layer with glyceride
trioleate being not more than 5.degree..
16. The method of claim 14 wherein said hydrophobic region has an
area of about 100 to 10,000 nm.sup.2.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydrophilic member which has an
amphiphilic surface, that is, a surface having both hydrophilicity
and hydrophobicity, and can permanently maintain this nature.
2. Background Art
A part of the present inventors has previously proposed a method
for highly hydrophilifying the surface of articles by
photoexcitation of a semiconductor photocatalyst (WO 96/29375 and
WO 97/23572). According to this method, the surface of articles can
be highly hydrophilified to a contact angle of the surface with
water up to about 0.degree..
When this method is applied to, for example, transparent articles
such as windshields for vehicles, door mirrors, windowpanes for
buildings, eyeglass lenses, or mirrors, the surface thereof is
highly hydrophilified, preventing the surface of the articles from
being fogged by moisture condensate or steam or from being blurred
by water droplets adhering on the surface thereof. Further, when
the method is applied to buildings or articles which are disposed
outdoors, oil repellent or hydrophobic dust and contaminants
adhering on the hydrophilified surface are easily washed away by
raindrops, thus permitting the surface to be cleaned.
SUMMARY OF THE INVENTION
The present inventors have clarified the state of the surface
hydrophilified by the application of the method described in WO
96/29375 and WO 97/23572 and found that the hydrophilic surface has
hydrophobic nature as well and that the properties of the
hydrophilified surface can be further improved. The present
invention has been made based on such finding.
Accordingly, an object of the present invention is to provide a
hydrophilic member which has an amphiphilic (hydrophilic and
hydrophobic) surface and can permanently maintain this
property.
Another object of the present invention is to provide a process for
producing a member having an amphiphilic surface and a method for
amphiphilifying (hydrophilifying and hydrophobifying) the surface
of a member.
The hydrophilic member according to one aspect of the present
invention comprises a substrate and a layer, containing a
photocatalyst, provided on the substrate. The surface of the layer
containing a photocatalyst has a hydroxyl group fixed thereon upon
photoexcitation of the photocatalyst. Furthermore, the surface of
the layer containing a photocatalyst has a water molecule
physically adsorbed in the vicinity of the hydroxyl group upon
photoexcitation of the photocatalyst.
The hydrophilic member according to another aspect of the present
invention comprises: a substrate; and a layer, containing a
photocatalyst, provided on the substrate, the photocatalyst
comprising a metal oxide, only oxygen atoms at bridging sites in
the metal oxide being substantially exposed on the surface of the
layer.
The process for producing a member having an amphiphilic surface
according to the present invention comprises the steps of:
providing a substrate; and forming a layer containing a
photocatalyst on the substrate.
The method for amphiphilifying the surface of a member according to
the present invention comprises the steps of: providing a
substrate; forming a layer containing a photocatalyst on the
substrate; and photoexciting the photocatalyst to amphiphilify the
surface of the layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a state that, upon
photoexcitation, a metal atom in a photocatalytic oxide is reduced
to create a vacancy lacking in oxygen and the vacancy is reacted
with a water molecule to fix a hydroxyl; group with the oxygen atom
filling up the vacancy;
FIG. 2 is a schematic diagram showing a state that a water molecule
is physically adsorbed onto the hydroxyl group, shown in FIG. 1,
through a hydrogen bond;
FIG. 3 is a schematic diagram showing a state that, in the surface
of the member according to the present invention, a hydrophilic
portion, where a hydroxyl group and a physically adsorbed water
molecule are present and a hydrophobic portion, where the hydroxyl
group and the physically adsorbed water molecule are absent, are
arranged in a mosaic form, wherein an open portion 1 represents the
hydrophilic portion and a black portion 2 represents the
hydrophobic portion;
FIG. 4 is a diagram showing the atomic structure of a metal oxide
as a photocatalyst, wherein M represents a metal atom and an oxygen
atom 11 is located in a position linking metal atoms to each other
and, in addition, is exposed on the surface of the metal oxide,
this oxygen atom being defined as an oxygen atom located at a
bridging site;
FIG. 5 is a diagram of the (110) crystal face, as viewed from
above, in a rutile titanium oxide crystal;
FIG. 6 is a diagram of the (100) crystal face, as viewed from
above, in a rutile titanium oxide crystal;
FIG. 7 is a diagram of the (001) crystal face, as viewed from
above, in a rutile titanium oxide crystal;
FIG. 8 is a graph showing a change in contact angle of the surface
of a photocatalytic titanium oxide layer with water upon
irradiation of the surface of the photocatalytic titanium oxide
layer with ultraviolet light having different wavelengths as a
function of light irradiation time, this change having been
determined in Example A1, wherein FIG. 8(A) shows data for
irradiation with light having a wavelength of 313 nm, FIG. 8(B)
data for irradiation with light having a wavelength of 365 nm, and
FIG. 8(C) data for irradiation with light having a wavelength of
405 nm;
FIG. 9 is a graph showing the results of an infrared spectroscopic
analysis of a titanium oxide disk in Example A2, wherein FIG. 9(A)
shows data for wave numbers 4000 to 2000 and FIG. 9(B) data for
wavenumbers 2000 to 1200;
FIG. 10 is a graph showing the results of an infrared spectroscopic
analysis, in Example B3, of a thin layer of anatase titanium oxide
provided on a glass substrate with gold vapor-deposited thereon;
and
FIG. 11 is a graph showing the relationship between the contact
angle of the (110) crystal face, the (100) crystal face, and the
(001) crystal face of a rutile titanium oxide single crystal with
water as a function of light irradiation time, the contact angle
having been, measured in Example C.
DETAILED DESCRIPTION OF THE INVENTION
The member according to the present invention is fundamentally
based on a hydrophilic member described in WO 96/29375 and WO
97/23572, which are incorporated herein by reference.
Mechanism of Hydrophilification and Surface State
In a member with a layer containing a photocatalyst ("a
photocatalyst-containing layer") formed thereon, photoexcitation of
the photocatalyst permits the surface of the member to be highly
hydrophilified. This is considered to proceed through the following
mechanism. In particular, irradiation of a photocatalyst with light
having higher energy than the band gap between the upper end of the
valence band and the lower end of the conduction band in the
photocatalyst results in excitation of electrons to create
conduction electrons and holes either or both of which probably
function to impart polarity to the surface. This permits a hydroxyl
group to be adsorbed onto the surface and, in addition, a
physically adsorbent water molecule to be fixed through a hydrogen
bond onto the substrate in its surface adjacent to the hydroxyl
group.
More specifically, the hydrophilification upon photoexcitation of
the photocatalyst is considered to proceed as follows. At the
outset, irradiation of the photocatalyst with excitation light
causes a metal atom in the photocatalytic oxide to be reduced. For
example, in the case of titanium oxide, tetravalent (+4) titanium
is reduced to trivalent (+3) titanium to create a vacancy lacking
in oxygen on the surface. This vacancy is reacted with a water
molecule to fix a hydroxyl group in such a manner that the oxygen
atom fills up the vacancy. FIG. 1 illustrates this state. It is
considered that a water molecule is then physically adsorbed onto
the hydroxyl group through a hydrogen bond. Further, another water
molecule is physically adsorbed to the physically adsorbed water
molecule, so that the surface exhibits high hydrophilicity. FIG. 2
shows the state of the physically adsorbed water molecule.
The surface which has been hydrophilified through this mechanism
can be permanently maintained so far as the surface is irradiated
with light to cause photoexcitation of the photocatalyst. Even
after the member according to the present invention is kept in the
dark, the hydrophilicity is returned upon light irradiation to
photoexcite the photocatalyst. According to some experiments
conducted by the present inventors, the following phenomenon was
observed. When a once hydrophilified member containing titanium
oxide as a photocatalyst according to the present invention is kept
in the dark, as is shown by the following chemical formula, the
surface is reacted with oxygen to reduce the amount of the hydroxyl
group chemically adsorbed onto the surface and oxygen is
coordinated to titanium atoms present on the surface. ##STR1##
When light is again applied to the surface of the member in the
presence of a water molecule to photoexcite the photocatalyst, the
coordinated oxygen is locally cleaved through the following
reaction. In the cleaved portion, a hydroxyl group is again bonded
to the titanium atom to form a hydrophilic portion. ##STR2##
Interestingly, it was found that, for the chemical adsorption of a
hydroxyl group and the physical adsorption of a water molecule
above, the whole surface of the photocatalyst-containing layer is
not simultaneously hydrophilified. Further, it was found that the
hydrophilification through the mechanism is locally created and the
number of hydrophilified sites and the hydrophilified area are
gradually increased, thus permitting the hydrophilification of the
surface to proceed. Furthermore, it was found that the contact
angle of the surface with water reaches zero degree before the
whole surface is hydrophilified.
In addition, interestingly, it was found that, in the member
according to the present invention, the area remaining
unhydrophilified is hydrophobic, that is, the hydrophilified member
according to the present invention serves also as a hydrophobic
member. Further, the hydrophobicity increased with increasing the
hydrophilicity of the surface. This means that the hydrophobicity
of the surface of the member increases simultaneously with
hydrophilification to a high extent. This will be exemplified in
detail. When the member according to the present invention is kept
in the dark, the amount of the hydroxyl group is reduced as
described above. The surface of the member in this state exhibits a
hydrophobicity of about 10 degrees in terms of contact angle
thereof with glyceride oleate. When light is again applied to
photoexcite the photocatalyst, the hydrophobicity, of the surface,
in terms of the contact angle with glyceride oleate can be lowered
to not more than 10 degrees, for example, not more than 5 degrees,
preferably 0 degree. At that time, the degree of hydrophilification
of the surface of the member is such that the contact angle of the
surface of the member with water is preferably not more than 5
degrees, more preferably 0 degree.
According to the present invention, therefore, an amphiphilic
member is provided wherein, in the surface of a
photocatalyst-containing layer, the area, where a hydroxyl group
and a physically absorbed water molecule are present, is
hydrophilic, while the area, where the hydroxyl group and the
physically absorbed water molecule are absent, is hydrophobic. On
the surface of the member, the hydrophilic portion and the
hydrophobic portion are present in a mosaic form. FIG. 3
schematically shows the surface of this member. In the drawing, an
open portion 1 represents the hydrophilic portion and a black
portion 2 represents the hydrophobic portion. According to a
preferred embodiment of the present invention, the hydrophilic
portion and the hydrophobic portion each have an area of about
10.sup.2 to 100.sup.2 nm.sup.2. The reason why the hydrophobicity
increases with increasing the hydrophilicity has not been
elucidated yet. However, it is considered that, as compared with
the case where the whole surface is homogeneously hydrophobic, the
presence of a hydrophobic surface in a mosaic form as shown in FIG.
3 permits oil droplets to be rapidly and homogeneously spread on
the surface of the member by virtue of a two-dimensional capillary
phenomenon.
Depending upon the partial pressure of water/oil in an atmosphere,
either component present in a larger amount preferentially has
affinity for the surface of the member wherein the hydrophilic
portion and the hydrophobic portion coexist. In particular, even
when the oil component is deposited on the surface of the member,
the deposited oil component can be easily removed by rinsing the
surface with a large amount of water. On the other hand, when water
is deposited on the surface of the member, water can be removed by
rinsing the surface with a large amount of an oil solvent.
The present inventors have further found that the rate of
hydrophilification upon photoexcitation varies depending upon the
crystal face of the photocatalyst. When the photocatalyst is a
metal oxide, the oxygen atom located at a bridging site in the
metal oxide is mainly deficient to create a vacancy. The oxygen
atom located at the bridging site-will be described with reference
to FIG. 4. FIG. 4 schematically shows the atomic structure of a
metal oxide as a photocatalyst. In the drawing, M represents a
metal atom, and oxygen atoms are bonded to the metal atom. An
oxygen 11 is located in a position linking metal atoms to each
other and, in addition, is exposed on the surface of the metal
oxide. An oxygen atom 12 is bonded to one metal atom only and is
exposed on the surface of the metal oxide, and an oxygen atom 13 is
located in a position linking metal atoms to each other and, in
addition, located within the crystal. In the present invention, the
oxygen atom 11 is defined as an oxygen atom located at the bridging
site.
When the member according to the present invention is constructed
so that the oxygen atom alone located at the bridging site is
substantially exposed on the surface of the
photocatalyst-containing layer, the hydrophilification rate is
improved. More specifically, the exposure of the oxygen atom
located at the bridging site onto the surface of the metal oxide
may be performed by regulating the crystal face(s), of the metal
oxide, to be exposed on the surface. For example, among the (110)
crystal face, the (100) crystal face, and the (001) crystal face in
a rutile titanium oxide crystal, the (110) crystal face and the
(100) crystal face are crystal faces on the surface of which the
oxygen atom located at the bridging site is exposed. FIGS. 5 and 6
are diagrams showing the (110) crystal face and the (100) crystal
face in the rutile titanium oxide crystal as viewed from above. In
these two crystal faces, the oxygen atom 11 located at the bridging
site is exposed on the surface. On the other hand, FIG. 7 shows the
(001) crystal face in the rutile titanium oxide crystal. In this
crystal face, all the oxygen atoms are located in a deeper position
than the titanium atom 14 and are not exposed on the surface of the
crystal face. Therefore, according to the present invention, when
the rutile titanium oxide crystal is used, use of the (110) crystal
face and the (100) crystal face, on the surface of which the oxygen
atom located at the bridging site is exposed, as the surface of the
member is preferred.
On the other hand, in the anatase titanium oxide. crystal, since
the crystal face, on the surface of which the oxygen atom located
at the bridging site is exposed, is the (001) crystal face, use of
this crystal face as the surface of the member is preferred.
Applications of Member
The member according to the present invention, by virtue of the
high hydrophilicity and hydrophobicity, can be utilized in various
applications.
Applications utilizing the hydrophilicity include those described
in WO 96/29375 and WO 97/23572, which are incorporated herein by
reference.
Applications utilizing both the hydrophilicity and the
hydrophobicity include those where the member gets intimate with an
oil and the oil component is easily removed. For example, in the
field of industrial equipment, machine tools, bearings, and cutting
disks, high lubricity of the oil during use and easy removal of the
oil component at the time of washing are often required. Although
the member according to the present invention gets highly intimate
with the oil, the deposited oil component can be easily removed by
washing. Thus, the member according to the present invention can be
preferably used in the above applications.
Photocatalyst-containing Layer
The term "photocatalyst" used herein refers to a material which,
when exposed to light (excitation light) having higher energy
(i.e., shorter wavelength) than the energy gap between the
conduction band and the valence band of the crystal, can cause
excitation (photoexcitation) of electrons in the valence band to
produce a conduction electron and a hole. Specific examples thereof
include, for example, anatase titanium oxide, rutile titanium
oxide, zinc oxide, ferric oxide, dibismuth trioxide, tungsten
trioxide, and strontium titanate. Further, photocatalysts described
in WO 96/29375 and WO 97/23572 are also usable in the present
invention, and the disclosure of these publications is incorporated
herein by reference.
Light sources which may be favorably used for photoexcitation of
the photocatalyst include sunlight, room lamps, fluorescent lamps,
incandescent lamps, metal halide lamps, mercury lamps, and xenon
lamps.
In the present invention, in order to highly hydrophilify the
surface upon photoexcitation of the photocatalyst the intensity of
the excitation light is preferably not less than 0.001 mW/cm.sup.2,
more preferably 0.01 mW/cm.sup.2, most preferably not less than 0.1
mW/cm.sup.2.
In the present invention, the photocatalyst-containing layer may
comprise silica or silicone besides the photocatalyst. Addition of
silica or a silicone can offer an advantage that, even when the
member according to the present invention is kept in the dark, the
hydrophilicity can be maintained for a long period of time.
In the present invention, the photocatalyst-containing layer may
further contain silver, copper, or zinc. Addition of these metals
enables bacterial and mold deposited onto the surface of the member
according to the present invention to be killed even when the
member is kept in the dark, offering an advantage that the surface
can be always kept clean.
Further, according to a preferred embodiment of the present
invention, platinum group elements, such as platinum, palladium,
ruthenium, rhodium, iridium, and osmium, may be added to the
photocatalyst-containing layer. Addition of these metals can
enhance the redox activity of the photocatalyst and can improve the
deodorant and cleaning effect.
In the present invention, the thickness of the
photocatalyst-containing layer may be suitably determined so far as
the effect of the photocatalyst can be attained. According to a
preferred embodiment, the layer thickness is not more than 0.4
.mu.m. This layer thickness can prevent opacification attributable
to diffused reflection of light, so that the surface layer becomes
substantially transparent. The layer thickness is more preferably
not more than 0.2 .mu.m. This thickness can prevent color
development of the layer attributable to the interference of light
and can offer improved transparency. Further, reduced layer
thickness advantageously results in improved abrasion resistance of
the layer.
Preparation
The member according to the present invention can be prepared by
forming the photocatalyst-containing layer on the surface of a
member to be served as a suitable substrate.
The photocatalyst-containing layer may be produced according to the
disclosure in WO 96/29375 and WO 97/23572, which are incorporated
herein by reference.
According to a preferred embodiment, when the
photocatalyst-containing layer consists of anatase titanium oxide
alone, preferably, it may be produced by a sol coating/annealing
method, an organotitanate method, or an electron beam vapor
deposition method.
(1) Sol Coating/annealing Method
An anatase titanium oxide sol is coated onto the surface of a
substrate by spray coating, dip coating, flow coating, spin
coating, roll coating or the like, and the coating is then
annealed.
(2) Oragnotitanate Method
A hydrolysis inhibitor (such as hydrochloric acid or ethylamine) is
added to an organotitanate, such as a titanium alkoxide (such as
tetraethoxytitanium, tetramethoxytitanium, tetrapropoxytitanium, or
tetrabutoxytitanium), titanium acetate, or a titanium chelate, and
the mixture is diluted with a nonaqueous solvent, such as an
alcohol (such as ethanol, propanol, or butanol). While partially
progressing hydrolysis or after complete hydrolysis, the mixture is
coated by spray coating, dip coating, flow coating, spin coating,
roll coating or the like, and the coating is then dried. The drying
results in completion of the hydrolysis to produce titanium
hydroxide, and an amorphous titanium oxide layer is formed on the
surface of the substrate by dehydropolycondensation of titanium
hydroxide. Thereafter, the layer is then annealed at a temperature
of the crystallization temperature of anatase or above to develop
phase transition of the amorphous titanium oxide to anatase
titanium oxide.
(3) Electron Beam Deposition
A titanium oxide target is irradiated with an electron beam to form
a layer of amorphous titanium oxide on the surface of a substrate.
Thereafter, the layer is annealed at a temperature of the
crystallization temperature of anatase to develop amorphous
titanium oxide to anatase titanium oxide.
When the photocatalyst-containing layer is formed of anatase
titanium oxide and silica, the member of the present invention may
be preferably produced, for example, by a sol coating/annealing
method, an organotitanate method, or a tetrafunctional silane
method.
(1) Sol Coating/annealing Method
A mixed solution composed of an anatase titanium oxide sol and a
silica sol is coated by spray coating, dip coating, flow coating,
spin coating, roll coating or the like, and the coating is then
annealed.
(2) Organotitanate Method
A hydrolysis inhibitor (such as hydrochloric acid or ethylamine)
and a silica sol are added to an organotitanate, such as a titanium
alkoxide (such as tetraethoxytitanium, tetramethoxytitanium,
tetrapropoxytitanium, or tetrabutoxytitanium), titanium acetate or
a titanium chelate, and the mixture is diluted with a nonaqueous
solvent, such as an alcohol (such as ethanol, propanol, or
butanol). While partially progressing hydrolysis or after complete
hydrolysis, the mixture is coated by spray coating, dip coating,
flow coating, spin coating, roll coating or the like, and the
coating is then dried. The drying results in completion of the
hydrolysis to produce titanium hydroxide, and an amorphous titanium
oxide layer is formed on the surface of the substrate by
dehydropolycondensation of titanium hydroxide. Thereafter, the
layer is then annealed at a temperature of the crystallization
temperature of anatase or above to develop phase transition of the
amorphous titanium oxide to anatase titanium oxide.
(3) Tetrafunctional Silane Method
A mixture of a tetraalkoxysilane (such as tetraethoxysilane,
tetrapropoxysilane, tetrabutoxysilane, or tetramethoxysilane) with
an anatase titanium oxide sol is coated onto the surface of a
substrate by spray coating, dip coating, flow coating, spin
coating, roll coating or the like, and, if necessary, after
hydrolysis to form a silanol, the silanol is dehydropolycondensed
by heating or the like.
When the photocatalyst-containing layer is formed of anatase
titanium oxide and a silicone, the member according to the present
invention may be produced as follows. Specifically, a coating of a
silicone or a silicone precursor is mixed with an anatase titanium
oxide sol, and, if necessary, after hydrolysis of the precursor,
the mixture is coated on the surface of a substrate by spray
coating, dip coating, flow coating, spin coating, roll coating or
the like, followed by heating or the like to dehydropolycondensate
the hydrolyzate of the silicone precursor to form a surface layer
formed of anatase titanium; oxide particles and a silicone. Upon
photoexcitation of anatase titanium oxide by irradiation with light
including an ultraviolet radiation, at least a part of organic
groups bonded to the silicon atom in the silicone molecule is
substituted by a hydroxyl group, and a physically adsorbed water
layer is further formed thereon, thus permitting the surface layer
thus to exhibit high hydrophilicity.
In this case, preferred examples of silicone precursors include
methyltrimethoxysilane, methyltriethoxysilane,
methytributoxysilane, methyltripropoxysilane,
ethyltrimethoxysilane, ethyltriethoxysilane, ethyltributoxysilane,
ethyltripropoxysilane, phenyltrimethoxysilane,
phenyltriethoxysilane, phenyltributoxysilane,
phenyltripropoxysilane, dimethyldimethoxysilane,
dimethyldiethoxysilane, dimethyldibutoxysilane,
dimehyldipropoxysilane, diethyldimethoxysilane,,
diethyldiethoxysilane, diethyldibutoxysilane,
diethyldipropoxysilane, phenylmethyldimethoxysilane,
phenylmethyldiethoxysiane, phenylmethyldibutoxysilane,
phenylmethyldpropoxysilane,
.gamma.-glycidoxypropyltrimethoxysilane, and hydrolyzates thereof
and mixtures thereof.
When exposing of the oxygen atom located at the bridging site is
contemplated, the photocatalytic oxide is fixed in a
crystallographically oriented state onto the surface of the
substrate. Methods usable for this purpose include, for example,
hot press annealing at 400.degree. C. or above after the ion beam
deposition (Journal of Material Research, vol. 2, No. 2 (1987))
The member to be served as the substrate may be suitably determined
by taking applications into consideration, and specific examples of
substrates include those described in WO 96/29375 and WO 97/23572,
which are incorporated herein by reference.
Among others, the following substrates are preferred in the present
invention.
When the member is expected to have antifogging effect, examples of
substrates usable herein include: mirrors, such as rearview mirrors
for vehicles, bathroom mirrors, lavatory mirrors, dental mouth
mirrors, reflecting mirrors for roads; lenses, such as eyeglass
lenses, optical lenses, lenses for cameras, lenses for endoscopes,
lighting lenses, lenses for semiconductors, and lenses for copying
machines; prisms; windowpanes for building or observation;
windowpanes for vehicles, such as automobiles, railway vehicles,
aircrafts, watercrafts, submarines, snowmobiles, ropeway gondolas,
pleasure garden gondolas and spacecrafts; windshields for vehicles,
such as automobiles, railway vehicles, aircrafts, watercrafts,
submarines, snow cars, snowmobiles, motorcycles, ropeway gondolas,
pleasure garden gondolas and spacecrafts; goggles for protection,
goggles for sports, shields of masks for protection, shields of
masks for sports, shields of helmets, glasses of display case for
frozen foods, and glasses of display cases for thermally kept
foods; covers for measuring instruments; and films for application
onto the above articles. These are required to be transparent, and
materials therefor include glass and plastics.
When the member is expected to have surface cleaning effect,
examples of substrates usable herein include building materials,
exterior of buidlings, interior of buildings, sashes, windowpanes,
structural members, exterior and coating of vehicles, exterior of
machineries and articles, dustproof covers and coatings, traffic
signs, various display devices, advertising towers or poster
columns, noise barriers for roads, noise barriers for rail roads,
bridges, exterior and coating of guard rails, interior facing and
coating of tunnels, insulators, cover for solar cells, covers for
solar energy collectors of solar water heaters, vinyl plastic
hothouses, covers for lighting of vehicles, households, stools,
bath tubs, wash basins, lighting equipment, covers for lighting,
kitchenwares, tablewares, dishwashers, dishdryers, sinks, cooking
ranges, kitchen hoods, ventilation fans, and films for application
on the surface of the above articles. Materials thereof include
metals, ceramics, glasses, plastics, woods, stones, cements,
concretes, fibers, woven fabrics, and combinations of the above
materials and laminates of the above materials.
Further, when the member is expected to have antistatic effect,
examples of substrates usable herein include: cathode-ray tubes;
magnetic recording media; optical recording media; photomagnetic
recording media; audio tapes; video tapes; analog records;
housings, components, exterior and coatings of domestic electric
appliances; housings, components, exterior and coatings of office
automation equipment; building materials; exterior of the
buildings; interior of the buildings; sashes; windowpanes;
structural members; exterior and coating of vehicles; exterior of
machineries and articles; dustproof covers and coatings; and films
for application onto the surface of the above articles. Examples of
materials therefor include metals, ceramics, glasses, plastics,
woods, stones, cements, concretes, fibers, woven fabrics, and
combinations of the above materials and laminates of the above
materials.
EXAMPLES
Example A1
Influence of Excitation Wavelength
A titanium oxide (anatase form) sol (STS-11, manufactured by
Ishihara Sangyo Kaisha Ltd.) was spray-coated on the surface of a
glazed tile having a size of 5.times.10 cm square (AB02E01,
manufactured by TOTO, LTD.), and the coating was fired for 10 min
at 800.degree. C. to prepare a sample A1.
For comparison, a glazed tile not having a titanium oxide coating
was allowed to stand in the dark for 10 days. These samples were
tested as follows.
The sample A1 and the comparative sample were irradiated with
monochromatic ultraviolet light using an Hg-Xe lamp under
conditions specified in the following Table 1, and a change in
contact angle with water as a function of the irradiation time was
determined. In this case, the contact angle of the, samples with
water was measured, with a contact angle goniometer (Model CA-X150,
manufactured by Kyowa Interface Science Co., Ltd.), 30 sec after
dropping a water droplet through a microsyringe on the surface of
the sample.
TABLE 1 Wavelength Irradiation inten- Density of of UV light sity
of UV light photon (nm) (mW/cm.sup.2) (photon/sec/cm.sup.2) 313
10.6 1.66 .times. 10.sup.16 365 18 3.31 .times. 10.sup.16 405 6
1.22 .times. 10.sup.16
The results are shown as graphs in FIGS. 8(A) to 8(C). In these
graphs, values plotted by open circles represent the contact angle
of the sample A1 with water, and values plotted by closed circles
represent the contact angle of the comparative glazed tile, not
coated with titanium oxide, with water.
As can be seen from the graph shown in FIG. 8(C), the irradiation
of the sample with ultraviolet light having lower energy than that
at a wavelength of 387 nm corresponding to the band gap energy of
the anatase titania (i.e., ultraviolet light having a longer
wavelength than 387 nm) results in no hydrophilification. On the
other hand, as shown in graphs of FIGS. 8(A) and 8(B), in the case
of irradiation of the sample with ultraviolet light having higher
energy than the band gap energy of the anatase titania, the surface
of the sample is hydrophilified in response to the ultraviolet
light irradiation. From the results, it has been confirmed that the
hydrophilification of the surface does not occur without the
photoexcitation of the semiconductor photocatalyst and is
attributable to the photocatalytic action.
Example A2
Adsorption of Hydroxyl Group and Physical Adsorption of Water
Molecule by Photoexcitation
A titanium oxide (anatase form) powder (P-25, manufactured by
Nippon Aerosil Co., Ltd.) was pressed to prepare disk samples. The
surface of these samples was investigated by Fourier transform
infrared spectroscopy (FT-IR) using a Fourier transform infrared
spectrometer (FTS-40A).
In each test, an ultraviolet lamp (UVL-21) at a wavelength of 366
nm was used for ultraviolet irradiation. In the analysis of the
infrared absorption spectrum, the following absorption band peaks
having the following wavenumbers appear and provide the following
information. Sharp absorption band at wavenumber 3690 cm.sup.-1 :
stretching of OH of hydroxyl group Broad absorption band at
wavenumber 3300 cm.sup.-1 : stretching of OH bond in physically
adsorbed water Sharp absorption band at wavenumber 1640 cm.sup.-1 :
bending of HOH bond in physically adsorbed water
At the outset, the titanium oxide disk immediately after pressing
was analyzed by infrared spectroscopy. An absorption spectrum for
the disk immediately after pressing is shown as a curve #1 in FIGS.
9(A) and 9(B).
The titanium oxide disk was stored for 17 hr in a dry box
containing silica gel as a desiccant and then analyzed by infrared
spectroscopy to provide an infrared absorption spectrum. The
absorption spectrum thus obtained is shown as a curve #2 in FIGS.
9(A) and 9(B).
Comparison of the spectrum #1 with the spectrum #2 shows that, for
#2, a dramatic reduction in absorption is observed at wavenumber
3690 cm.sup.-1, indicating a reduction in hydroxyl group
(chemically adsorbed water). Further, for #2, a dramatic reduction
in absorption is observed also at wavenumbers 3300 cm.sup.-1 and
1640 cm.sup.-1, indicating that the physically adsorbed water as
well has been reduced. Thus, it is apparent that storage in dry air
resulted in a reduction in both chemically adsorbed water and
physically adsorbed water. In general, when the disk is allowed to
stand in the dark, the contact angle of the disk to water is
increased, suggesting that the surface of the disk has been
rendered somewhat hydrophobic.
The titanium oxide disk was then placed in the dry box and
irradiated with ultraviolet light at an irradiation intensity of
about 0.5 mW/cm.sup.2 for one hr, followed by infrared
spectroscopic analysis of the surface of the disk to provide an
infrared absorption spectrum. The absorption spectrum thus obtained
is shown as a curve #3 in FIGS. 9(A) and 9(B). As can be seen from
the absorption spectrum #3, the absorption at wavenumber 3690
cm.sup.-1 returned to substantially the same level of absorption as
observed in the initial state. Further, the absorption at
wavenumbers 3300 cm.sup.-1 and 1640 cm.sup.-1 also returned to the
same level of absorption as observed in the initial state. These
results show that ultraviolet irradiation brings both the amount of
the hydroxyl group and the amount of the physically adsorbed water
to those observed in the original state.
As is apparent from the results of Example A1, since ultraviolet
irradiation results in lowered contact angle of the surface of
titanium oxide particles with water, it is considered that, also in
this case, the surface thereof has been hydrophilified.
Example A3
Investigation of Form of Adsorption of Hydroxyl Group and
Physically Adsorbed Water
The surface state of the (110) face (crystal face, with the highest
atom density, having such a structure that oxygen atoms
crosslinking titanium are arranged in the direction of c axis) was
investigated by atomic force microscopy (AFM).
At the outset, for the rutile single crystal, the contact angle of
the (110) face with water was measured and found to be 63.degree..
Further, for this sample, the (110) face was observed under an
atomic force microscope. As a result, only a smooth face was found,
and no protrusion attributable to the adsorption of a hydroxyl
group and physically adsorbed water was found, demonstrating that
the surface of the rutile single crystal is hydrophobic in the
original state.
Ultraviolet light was applied at 1 mW/cm.sup.2 to the (110) face
for 42 hr. Thereafter, the contact angle of the (110) face with
water was measured. As a result, it was found that the (110) face
was hydrophilified to 37.degree. in terms of the contact angle. For
this sample, the (110) face was observed under an atomic force
microscope. As a result, a waterdrop pattern in a mosaic form was
recognized. The proportion of the area occupied by the waterdrop
pattern was determined and found to be 22%. Further, observation
using a hydrophilic probe demonstrated that the waterdrop pattern
was made of water, suggesting that the waterdrop pattern observed
herein is composed of a hydroxyl group and physically adsorbed
water.
As a result of observation of the waterdrop pattern portion in a
further enlarged state, it was found that a hill-like protrusion
portion is present adjacent to a thin striped protrusion portion.
The size of the striped protrusion portion and the hill-like
protrusion portion was examined by atomic force microscopy. As a
result, the stripe protrusion portion had a height of 0.3 nm and a
width of 16 nm. Since the atomic distance of O--H in water is about
0.1 nm, it is considered that several hydroxyl groups adsorbed on
the stripe portion gather to form a hydrophilic domain. The
hill-like protrusion portion had a height of 7 nm and a width of 85
nm.
Nitrogen gas was then sprayed on the (110) face, and a change in
the hill-like protrusion portion was observed by atomic force
microscopy. As a result, it was found that the hill-like protrusion
disappeared, suggesting that the hill-like protrusion portion was
composed of physically adsorbed water. That is, the thickness of
the physically adsorbed water was judged to be, equal to the
hill-like protrusion portion and to be 7 nm.
Thus, it was demonstrated that the photoexcitation of a
photocatalyst resulted in adsorption of a hydroxyl group and
permitted physically adsorbed water to be adsorbed adjacent to the
hydroxyl group. Further, since the hydroxyl group is adjacent to
the physically adsorbed water, it is expected that, upon
photoexcitation of the photocatalyst, the hydroxyl group is first
adsorbed to form a domain and the physically adsorbed water is then
adsorbed by a hydrogen bond to the hydroxyl group.
Example B1
An anatase titanium oxide sol was coated onto a glass substrate by
spray coating, and the coating was annealed. at 500.degree. C. to
prepare sample B1 comprising a 0.3 .mu.m-thick thin layer of a
photocatalyst provided on the surface of a glass substrate. The
thin layer surface of the sample B1 had a contact angle with water
of 37.degree., that is, was hydrophilic.
The sample B1 was allowed to stand in the dark for 2 months to
prepare sample B2. The contact angle of the sample B2 with water
was measured and found to be increased to 72.degree.. That is, the
surface was hydrophobified. In this case, the contact angle of the
sample B2 with glyceride trioleate, which is a main component of a
food oil, was also measured and found to be 10.degree., indicating
that the sample B2 is hydrophobic.
Further, the sample B2 was irradiated with ultraviolet light having
a central wavelength of 360 nm at 1 mW/cm.sup.2 for 10 hr to
prepare sample B3. The contact angle of the sample B3 with water
was measured and found to be decreased to 0.degree., indicating
that the surface was superhydrophilified. In this case, the contact
angle of the sample B3 with glyceride trioleate, which is a main
component of a food oil, was also measured and found to be
decreased to 2.degree., indicating that the surface was
superhydrophobified.
Glyceride trioleate was then dropped onto the surface of the sample
B3, and the sample B3 was rinsed with water. As a result, glyceride
trioleate was easily removed.
Further, water was dropped on the surface of the sample B3, and the
sample B3 was rinsed in glyceride trioleate. As a result, water was
easily removed.
Example B2
A thin film of a rutile titanium oxide single crystal was allowed
to stand in the dark for 2 months to prepare sample B4. For the
sample B4, the contact angle of the (110) face with water was
measured and, as with the contact angle of the sample B2, was found
to be high, i.e. 63.degree., suggesting that the sample B2 and the
sample B4 have substantially the same microstructure.
The surface of the sample B4 was observed under an atomic force
microscope. As a result, a smooth face was observed. Thus, it was
confirmed that the surface of the sample was homogeneous.
Consideration of all the results of measurement of the contact
angle of the sample B2 on its surface with glyceride trioleate and
the results of the observation of the sample B4 on its surface
under an atomic force microscope has lead to a conclusion that the
surface of the sample B2 and the surface of the sample B4 had
homogeneous hydrophobicity.
Further, the surface of the sample B4 was irradiated with
ultraviolet light having a central wavelength of 360 nm at 1 mW/cm
for 42 hr to prepare sample B5. The contact angle of the surface of
the sample B5 with water was measured and found to be 37.degree.,
indicating that the surface was hydrophilified.
The surface of the sample B5 was observed under an atomic force
microscope. As a result, unlike the sample B4, a waterdrop-like
protrusion, in a mosaic form, of several tens of nm in size was
observed.
Example B3
An anatase titanium oxide sol was spray-coated onto a glass
substrate with gold vapor-deposited thereon, and the coating was
annealed at 500.degree. C. to form a 0.3 .mu.m-thick thin layer of
a photocatalyst on the surface of the glass substrate. The coated
substrate was used as sample B6.
The sample B6 was allowed to stand in the dark for 7 days to
prepare sample B7.
Further, the surface of the sample B7 was irradiated with
ultraviolet light having a central wavelength of 360 nm at 1
mW/cm.sup.2 for 5 hr to prepare sample B8.
The surface of the samples B6 to B8 was observed by Fourier
transform infrared spectroscopy (FT-IR) using a Fourier transform
infrared spectrometer (FTS-40A).
The results were as summarized in FIG. 10. As is apparent from FIG.
10, standing of the sample in the dark resulted in reduced
stretching of OH of hydroxyl group, stretching of OH bond in
physically adsorbed water, and bending of HOH bond in physically
adsorbed water. Irradiation of the surface of the samples with
ultraviolet light resulted in increased stretching of OH of
hydroxyl group, stretching of OH bond in physically adsorbed water,
and bending of HOH bond in physically adsorbed water. From the
facts, it is considered that standing of the samples in the dark
causes the hydroxyl group and the physically adsorbed water on the
surface to be reduced leading to hydrophobification, while
irradiation of the surface of the samples with ultraviolet light
causes the hydroxyl group and the physically adsorbed water on the
surface to be increased leading to hydrophilification.
When all the results of Examples B2 and B3 are taken into
consideration, it is considered that the waterdrop-like protrusion
portion in a mosaic form observed on the sample B4 is a hydrophilic
section constituted by the hydroxyl group and the physically
adsorbed water. It can be concluded that the waterdrop-like
protrusion portion is a portion formed upon exposure of the surface
of the sample B4 to ultraviolet light. Thus, the sample has such a
surface that a hydrophilic section and a hydrophobic section of
several tens of nm in size are dispersed in a mosaic form. It is
believed that the mosaic structure creates a two-dimensional
capillary phenomenon, leading to the development of amphiphilicity
(both hydrophilicity and hydrophobicity).
Example C
A rutile titanium oxide single crystal was provided, and the
contact angle of the (110) crystal face, the (100) crystal face,
and the (001) crystal face with water was measured with a contact
angle goniometer (CA-X150, manufactured by Kyowa Interface Science
Co., Ltd.). The contact angle was measured 30 sec after dropping a
water droplet through a microsyringe on the surface of the sample.
As a result, the contact angle was 85.degree. for the (110) crystal
face, 30.degree. for the (100) crystal face, and 84.degree. for the
(001) crystal face.
Each of the crystal faces was irradiated with ultraviolet light at
40 mW/cm.sup.2 from a mercury lamp to investigate a change in
contact angle of the crystal face with water as a function of the
irradiation time.
The results were as shown in FIG. 11. Specifically, as compared
with the (001) crystal face, the (110) crystal face and the (100)
crystal face were hydrbphilified at a higher rate in response to
the photoexcitation to a contact angle thereof with water of
0.degree..
* * * * *